"Undoubtedly, use is driven by a common belief that supplements can improve health and protect against disease, and that at worst, they are harmless.

"However, the assumption that any dietary supplement is safe under all circumstances and in all quantities is no longer empirically reasonable."

Health supplements are booming in the US, with annual sales estimated at 30 billion dollars (£18.6 billion), said the scientists, who assessed the evidence relating to several supplements including antioxidants, folic acid, vitamin D and calcium.

A number of animal, laboratory and observational studies had appeared to show that dietary supplements could lower cancer risk, they said. However, these findings were not confirmed by the "gold-standard" in evidence-based medicine, randomised controlled trials (RCTs).

Only a small number of RCTs had been carried out to test the effectiveness of dietary supplements, said the experts - and several of these had reported increased risks.

"Supplementation by exogenous anti-oxidants may well be a two-edged sword," the scientists wrote. "These compounds could, in vivo (outside the laboratory), serve as pro-oxidants or interfere with any number of protective processes such as apoptosis induction."

Experimental studies had shown that different tissues with different cancer-triggering pathways may not respond the same way to a particular nutrient.

"In fact, a nutrient may be associated with protection in one tissue and harm in another," said the experts.

They added that supplement users were "sometimes quick to discredit caution" and distrustful of mainstream science which they suspected of being corrupted by links to the drug industry.

Users may also assume the supplements they bought to be as well regulated as over-the-counter medications.

"These beliefs underscore the need for efforts by scientists and government officials to encourage the public to make
prudent decisions based on sound evidence with respect to use of dietary supplements for cancer prevention," the scientists concluded.

In a statement, Graham Keen, executive director of the The Health Food Manufacturers’ Association (HFMA) said: “The vitamin and mineral supplements industry has an exceptional record of both safety and efficacy, in the UK and worldwide.”

“Figures published by the Food Standards Agency showed that there were only 11 reported reactions to food supplements over an 11 year period, the majority of them in the lowest category of harm. Compared to other foods or medicines, food supplements have an enviable record.

The HFMA believe that research demonstrating fruit and vegetables have been depleted in minerals since the Thirties, means additional steps need to be taken.

“Alongside the fact that just 15% of women and 13% of men are eating the recommended five portions of fruit and vegetables, it is important that consideration be given to responsible supplementation for good health.”